Compression ratio question

[kenh]

Assuming a 350 with the 193 heads.

All things being equal, Going from the stock dished pistons to a flat top piston, what would be the expected compression ratio? Should mention the bore is .030 over in size.

I'm guessing some of you have done this and know the answer.

Thanks!!

Ken

 

[badco]

Same gasket, same deck height?

 

[badco]

So in perfect world that engine was supposed to be 9.2ish and a flattop make it 9.8-10;1. Heads are supposed to be 64 but usually 66-68 when actually cc them, depending on what brand of piston some cheap pistons are made to compensate for deck being cut and are in the hole a little further. There is alot of factors

 

[Erik the Awful]

https://speedmaster79.com/tools/engine-compression-ratio
With a 4.030" bore and 3.480" stroke, a 64cc combustion chamber, flat-tops with 7cc of valve reliefs, a .041 head gasket, and an undecked block putting the pistons .025" in the hole, you're looking at 9.57:1 compression ratio. Nearly the same specs as my low-buck build. If the combustion chambers actually measure out to 66cc, you're looking at 9.37:1.

 

[kenneth j hanawalt]

Thanks for the info

This short block I'm getting has flat top pistons in it. I dont want to be stuck burning premium gas. Should be safe with those ratios.

I don't know if the block has been decked or not.

Ken

 

[Schurkey]

When GM put "flat-top" pistons in a TBI engine using the ****** swirl-port heads, they claimed less than half-a-point more compression than the dish pistons that went into the trucks.

GM used flat-tops on the B-body (Caprice) TBI 5.7L '91--'93. Claimed 9.6 compression vs. 9.2 for the light-duty trucks.. I ran the service-replacement Caprice engine in my K1500 for almost twenty years, stock PROM and it ran flawlessly once I replaced the faulty O2 sensor.

The GM pistons apparently are short .010 in compression height. At least, the aftermarket replacements are short. Kinda sucks, they'd be a great piston otherwise, and they're dirt cheap.

If the short-block you're looking at is from a B- or D-body TBI, you'll have the roller lifters, too. Good engines. Measure the quench distance, use the thinnest head gasket that gets you at least .030--.035 quench.

 

[b454rat]

I ran a TBI 406 years ago with just about 10:1 compression on 87, no issues.

 

[abomb2016]

With proper quench space, you can run higher compression without pinging...

 

[kenh]

When GM put "flat-top" pistons in a TBI engine using the ****** swirl-port heads, they claimed less than half-a-point more compression than the dish pistons that went into the trucks.

GM used flat-tops on the B-body (Caprice) TBI 5.7L '91--'93. Claimed 9.6 compression vs. 9.2 for the light-duty trucks.. I ran the service-replacement Caprice engine in my K1500 for almost twenty years, stock PROM and it ran flawlessly once I replaced the faulty O2 sensor.

The GM pistons apparently are short .010 in compression height. At least, the aftermarket replacements are short. Kinda sucks, they'd be a great piston otherwise, and they're dirt cheap.

If the short-block you're looking at is from a B- or D-body TBI, you'll have the roller lifters, too. Good engines. Measure the quench distance, use the thinnest head gasket that gets you at least .030--.035 quench.

With proper quench space, you can run higher compression without pinging...

No clue if it's a D or B body block. I do know it has no provision for a fuel pump. All it has is the block, crank, rods and pistons.

With proper quench space, you can run higher compression without pinging...

I read, I think, someplace that .040" is about the ideal quench spacing???? Correct?

Ken

 

[Schurkey]

No clue if it's a D or B body block. I do know it has no provision for a fuel pump. All it has is the block, crank, rods and pistons.

No doubt there's identifying numbers/letters on the block that could tell you the origin.

The TBI B/D bodies got flat-tops. The TBI light trucks got dish pistons. I don't know about TBI F- or Y-body.

I read, I think, someplace that .040" is about the ideal quench spacing???? Correct?

I suspect that the actual square-inch area of the quench pad has an effect on "optimum" quench distance.

You need a minimum amount to prevent the pistons from hitting the valves on overlap. Proper diameter and depth valve notches or dish on the piston tops should be the real solution here. You need a minimum amount of distance to keep the piston top from hitting the underside of the cylinder head. That varies with RPM, connecting rod material (more space needed with aluminum rods) bore size, and piston-to-wall clearance (piston rock). I've heard of--but never seen with my own eyes--pistons that have light "witness marks" from touching the head at redline.

OTOH, anything over .045--.050 isn't tight enough to do any good. The real "danger zone" runs from about .050 to about .120--too tight to support combustion, but still leaves a substantial amount of end-gasses that get heated and detonate. Looser than .100--.120 allows enough space to support combustion--you have an "open" combustion chamber. It may burn slow, need a lot of advance for best power, but it's not as likely to detonate as a somewhat tighter chamber.

So, yeah, .040 is a good number to shoot for. "Zero deck" the block, use a common .039--.040 head gasket, and you're set. Or, cut a little less off the block, use a .028 head gasket.